The present invention relates to a clutch assembly and method for limiting torque transmission particularly applicable in an electrically energized starter for an internal combustion engine.
Electric starter motors are widely utilized for cranking small gasoline engines such as those utilized in garden tractors, lawn mowers, snow blowers, outboard motors for boats and the like. In such a starter, a pinion drive provides the means for momentarily engaging the engine flywheel to transfer power from the electric starting motor to the internal combustion engine and then disengaging the starter motor from the flywheel once the engine has started to prevent damage to the starter motor. The most common way to facilitate engagement and disengagement of the pinion with the flywheel is to mount the pinion gear to a shaft so that it is rotatably driven by the motor and is simultaneously moved axially along the shaft. The axial movement allows full engagement of the pinion gear with the flywheel during cranking and complete disengagement once the engine has started. The axial travel of the pinion gear is generally facilitated by one of two means. The pinion gear is either forced along the shaft by a solenoid or by inertia of the pinion gear interacting with the accelerating motor shaft by means of mating helical threads on the pinion gear and the associated shaft.
Exemplary starter assemblies are illustrated and described in U.S. Pat. Nos. 3,690,188 and 4,255,982.
In a typical starter assembly, the flywheel of the associated internal combustion engine has gear teeth formed about the outer periphery thereof and a spring biased pinion gear adapted to selectively drivingly engage the flywheel gear teeth is coupled to the output shaft of a starting motor through a torque limiting friction clutch and a helical spline.
When the starting motor is energized and commences to rotatingly drive the output shaft, the inertia of the pinion gear resists rotation and the helical spline causes the pinion gear to translate axially along the starting motor output shaft and thence into engagement with the gear teeth of the flywheel.
The engine is then cranked until the speed of the engine surpasses the speed at which it is driven by the starting motor. When the engine speed surpasses the starting motor speed, the helical spline causes the pinion gear to disengage from the flywheel gear teeth. Simultaneously, an associated anti-drift helical spring urges the pinion gear out of engagement and toward its normal rest position.
It is readily apparent that during the starting of an internal combustion engine, the starting motor including the associated pinion, is subjected to considerable shock and loading stresses as it initially engages and disengages from the flywheel gear teeth of the engine.
Such stresses are inherent as the starting motor armature and pinion are rotating as the pinion gear engages the relatively large mass of the engine flywheel and associated engine components which are at rest.
An ever present problem encountered in the design of an electric starting motor for cranking internal combustion engines is providing the starting motor components with means to absorb or decrease the torsional shock when the pinion gear of the starting motor initially engages the flywheel gear teeth of the associated engine. At the time of the engagement, the armature, drive shaft, and pinion are rotating at a relatively high speed and the flywheel of the engine is not rotating. The moment the pinion gear of the starting motor engages the flywheel gear teeth, a sudden torsional shock is imparted to the flywheel as well as to the starting motor pinion gear and associated armature. The resultant torsional shock may result in damage to either the starting motor or the flywheel, or both.
Various schemes over the years have been developed in an attempt to solve or minimize the problem. Some attempts have been directed to mechanisms to soften or decrease the torsional shock upon engagement, while other attempts have utilized a slip-clutch of some configuration.